Elizabeth Kolbert is a staff writer at The New Yorker and also the author of The Prophet of Love and Field Notes from a Catastrophe.

Barry GoldsteinHenry Holt & Co.

The dinosaurs were killed during the Fifth Extinction — which scientists suspect was caused by an asteroid. Now, we are living through an epoch that many scientists describe as the Sixth Extinction, and this time, human activity is the culprit. As one scientist put it: We're the asteroid.

Elizabeth Kolbert is the author of the new book The Sixth Extinction. It begins with a history of the "big five" extinctions of the past, and goes on to explain how human behavior is creating a sixth one — including our use of fossil fuels and the effects of climate change.

"We are effectively undoing the beauty and the variety and the richness of the world which has taken tens of millions of years to reach," Kolbert tells Fresh Air's Terry Gross. " ... We're sort of unraveling that. ... We're doing, it's often said, a massive experiment on the planet, and we really don't know what the end point is going to be."

Climate change was the subject of Kolbert's previous book, Field Notes from a Catastrophe. Her research for the new book took her around the world, to oceans, rain forests and mountains — as well as a place nearly in her backyard — where scientists are studying disappearing plants and animals.

"Amphibians have the dubious distinction of being the world's most endangered class of animals," she writes. "But also heading toward extinction are one-third of all reef-building corals, a third of all fresh-water mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles and sixth of all birds."

Interview Highlights

On how we define mass extinction

The definition, I suppose, would be many, many organisms across many, many different groups. And that is, really, what we are seeing and that is what makes scientists fear ... that we're in a mass extinction. ... About a quarter of all mammals are considered endangered. ... About 40 percent of all amphibians are considered endangered. But we're also seeing organisms, invertebrates, for example, are endangered ... many species of reef-building corals are now considered very, very endangered.

So you're seeing extinctions across a wide variety of groups, and that, I think, would have to be one of the defining characteristics of a mass extinction.

What happens when you put CO2 into the air is that you're also effectively pumping it into the oceans, because wherever the surface of the oceans and the atmosphere meet, there's just an exchange of gases. So about a third of the CO2 that we put up every year — and that's in the order of 10 billion metric tons — is making its way into the oceans. And when CO2 dissolves in water it forms an acid. It's called carbonic acid ... lowering the pH of the water. If you're a marine organism and ... your whole environment is the water and you change the chemistry of the water, that can have very, very profound effects. ...

[It's] global warming's equally evil twin ... and from the perspective of the broad expanse of life, there have been a few moments in time where the oceans have become acidified, not necessarily acidic but acidified ... and they are associated with some of the major crises in the history of life.

On a study that examined how CO2 emissions affected marine life

If we continue at our present rate of CO2 emissions, then by the end of this century ... [extrapolating from this study,] you're looking at eliminating a third of the creatures in the ocean as a very rough estimate. And then as you ... get closer and closer to [underwater gas vents] — so even beyond what we expect at the end of this century — if we sort of continue beyond that point, then you're getting to a point where your oceans really start to look sort of like the underwater equivalent of a vacant lot.

On the extinction of the Panamanian golden frog

There's a frog known as the Panamanian golden frog, and it's a beautiful, beautiful frog. It's sort of a taxicab-yellow frog with very skinny arms and legs and it's considered a good-luck symbol. ... These yellow frogs were just all over. ... And then this scourge came through, which turns out to be a fungus, a fungal disease, and just wiped them out. And what happened is that scientists realized — because they had seen this pattern before, they actually anticipated this — and they took some of them out of the forest into a conservation center ... and they are breeding them there. There are still Panamanian golden frogs in this center, but they cannot go back out into the world into their own native habitat because this fungus is still there, and the fungus survives even though the frogs no longer do.

On a theory of how the fungus spread

One theory — it has been very difficult to pin down — but it's that this fungus was moved around the world. Another really interesting story on frogs [is that they] were used in the '50s and '60s for pregnancy tests. Something called the African clawed frog, if you inject it with the urine of a woman who is pregnant, it will lay eggs very quickly. And obstetricians used to keep whole tanks of these frogs in their offices. And the African clawed frog turns out to be a frog that carries this fungus but doesn't seem to be killed by it. So one theory is that as these frogs were [exported] around the world, they carried this fungus with them ... so we brought the frogs and the frogs brought the fungus.

On how people have disrupted ecosystems by bringing plants and animals with them when they travel

Rats were brought with very early Polynesian settlers in the Pacifics, so they were brought to places like Fiji and they wreaked havoc. Already, say, 1,500 years ago. ...

I think what has changed is the scale and the rate. For example, it's been estimated that just in ship ballast now in our enormous supertankers, people are moving 10,000 species a day around the world. So even though our ancestors were already at this project and did cause significant effects because of that, and significant numbers of extinctions, we have just ramped it up to a whole new level.

On bats dying off

Bats just [started] dropping dead all of a sudden ... around 2007. It happen[s] that the epicenter of this was right near where I live ... in Western Massachusetts. This was first noticed in Upstate New York. Right as it happened I was able to go out with some scientists to a cave in Vermont, which was the largest bat hibernacula in New England.

So bats in the northeast hibernate in the winter, they go into a state of torpor, they hang by their toes. They try to find a place like a cave where the climate is going to remain pretty stable over the winter, and their body temperature drops almost to freezing, so you'll see ice crystals on them a lot of times. And they just hang there, completely motionless — it's an amazing thing to see. In this bat cave in Vermont, they were particularly hard hit. They just started to drop from the top of this cave to the floor in these huge drifts of dead bats. I went out in the winter of 2009, and it was an absolutely gruesome scene of a carpet of dead bats.

On how ecosystems recover from mass extinction

After a mass extinction, it has generally tended to take many millions of years for life to recover. It's not something that you bounce back from, from one day to the next.

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. We're living in an epoch many scientists describe as the sixth extinction. The fifth extinction was the one that killed off the dinosaurs. Scientists now think an asteroid was responsible for that mass extinction, but this time around, human activity is responsible, or as one scientist put it, this time we're the asteroid.

My guest, Elizabeth Kolbert, is the author of the new book "The Sixth Extinction." It begins with a history of the big five extinctions of the past and goes on to explain how human behavior is creating the sixth, including our use of fossil fuels, which has led to climate change. Climate change was the subject of Kolbert's previous book, "Field Notes from a Catastrophe."

Her research for the new book took her around the world to oceans, rainforests and mountains, as well as a place nearly in her backyard, where scientists are studying die-offs of plants and animals. Elizabeth Kolbert, welcome to FRESH AIR. So when we're talking about the sixth extinction, what are we talking about? What are the species that are likely to become extinct or that have already become extinct? What is dying off?

ELIZABETH KOLBERT: Well, when you're talking about an extinction event, the definition I suppose would be many, many organisms across many, many different groups. And, you know, that is really what we are seeing, and that is what make scientists fear, I suppose you'd say the word, that we're in a mass extinction. You're seeing mammals, about a quarter of all mammals are considered endangered, for example; about 40 percent of amphibians are considered endangered, but we're also seeing organisms - invertebrates for example, are endangered; for example reef-building corals, many species of reef-building corals are now considered very, very endangered.

So you're seeing extinctions across a wide variety of groups, and that I think would have to be, you know, one of the defining characteristics of a mass extinction.

GROSS: I think of carbon emissions as affecting the atmosphere. But you write about how it's also affecting the oceans and life within oceans. How are greenhouse gases affecting the acidity of oceans?

KOLBERT: Well that's (unintelligible) that I think is not as fully appreciated as it should be, and it's sort of one of the reasons, even, you could say that I wrote the book. And that is people are pretty aware now of what CO2 emissions, you know, tailpipe emissions do to the atmosphere. They warm the atmosphere. The Earth is getting warmer. There's, you know, no doubt about that.

But what happens when you put CO2 into the air is that you're also effectively pumping it into the oceans because wherever the surface of the oceans and atmosphere meet, there's just an exchange of gases. So about a third of the CO2 that we put up every year, and that's on the order of, you know, 10 billion metric tons, is making its way into the oceans.

And when CO2 dissolves in water, it forms an acid. It's called carbonic acid. It's a very weak acid, and you're drinking it, for example, when you drink a Coke. And it's that little bit of acidity that give soft drinks what's sometimes called their zest. You know, and it's one of the reasons we don't like to drink soft drinks that are flat, that they taste very treacley(ph).

But you do that on a massive enough scale, and you are changing the chemistry of the ocean. You're turning the water more and more acidic, for those who remember their high school chemistry lowering the pH of the water. And that's exactly what we're doing. And if you're a marine organism, and you think about it, you're in the water, you're completely - everything that - your whole environment is the water.

And then when you change the chemistry of the water, that can have very, very profound effects.

GROSS: Which is why you say ocean acidification is sometimes referred to as global warming's evil twin.

KOLBERT: Yeah, global warming's equally evil twin, exactly. And from the perspective of, you know, the broad expanse of life, there have been a few moments in time where the oceans have become acidified, not necessarily acidic but acidified, so more and more acidified, and they are associated with some of the major crises in the history of life.

GROSS: To help you understand the impact of the acidification of the oceans, you went to a tiny island that has naturally occurring gas bubbles, and there's carbon dioxide in those gas bubbles. So what kinds of experiments are going on in this island?

KOLBERT: Well that's a really interesting story, and it involves a great guy whom I went out with named Jason Hall-Spencer, who's a British marine biologist. And he came to this idea, just sort of almost by chance. He went swimming off of this tiny island, as you say, and it - which is in the Bay of Naples. It's off of Ischia in Italy.

And their CO2, this is an area of a lot of volcanic activity, and their CO2 just coming naturally out of the bottom of the water, just bubbling up. And people like to go swimming there because it's very cool, and it's like sort of swimming in fizzy water or champagne. And some people took him swimming there, and he realized wow, this is a natural experiment in ocean acidification. This is CO2 being pumped into the water from below, and it's very similar to what we're doing, sort of effectively pumping CO2 into the water from above.

And if I look at what's happening around here, I will be able to look -effectively look into the future and see what's going to happen if we continue to pump CO2 into the air. And so he did a census around this island of marine creatures as you were far away from these CO2 bubbles and as you got closer and closer. And what he found, and I went out with him, and we went swimming in this frigid water, and you see this amazing tableau where when you're far away from the vents, you're seeing a very vivid, you know, underwater world with sea urchins and barnacles and corals and all - fish and all sorts of things that you expect to see in the Mediterranean.

And then as you get closer and closer to the vents, you see less and less until you get to this landscape that looks really like a lunar landscape, where very, very little can survive. So this is sort of like this interesting underwater time machine where we can look into the future of the oceans.

GROSS: So if our oceans ended up looking like the area where there's these naturally occurring gas vents are, what would our oceans end up looking like?

KOLBERT: Well, his experiments suggest that if we continue, you know, at our present rate of CO2 emissions, then by the end of this century, ocean pH, ocean - will have dropped, or ocean acidity will have increased, depending on how you want to say it, to the point where roughly a third, in his - in this particular ecosystem roughly a third of the organisms drop out when you get to that pH.

So you're looking at eliminating a third of the creatures in the ocean for - as a very, very rough estimate. And then as you go on, as you get closer and closer to the vents, so even beyond, you know, what we expect at the end of this century if we just sort of continue beyond that point, then you're getting to a point where, you know, your oceans really start to look sort of like the underwater equivalent of a vacant lot.

GROSS: So, you know, if we just want to look at it in a very selfish way, this is going to affect what we eat.

KOLBERT: Well, I think that, you know, already, obviously, long before you get to sort of the end of this century and the effects of ocean acidification that he saw, we're already seeing tremendous effects to the ocean from a variety of causes, you know, overfishing, bottom trawling. Global warming is really changing where the oceans are warming very quickly. It's really changing where things can live, what things can live.

So yeah, we're definitely seeing changes to what we as people can eat, absolutely.

GROSS: Now you also went to the Great Barrier Reef, off the coast of Australia. What were you looking for there?

KOLBERT: Well, that was an amazing place, perhaps the most amazing place I've ever been in my life, and I also went with a great group of scientists there. A scientist out of Stanford named Ken Caldeira was running a series of experiments off of this tiny, tiny little island that just sort of pokes up out of the reef. You're essentially on the reef, and every 12 hours, basically at low tide, people would walk across the reed and just go to collect water samples, from which they were trying to tease out the question of how corals are responding to these changes in ocean chemistry.

And to walk across the Great Barrier Reef in the middle of the night is a wonderful, wonderful experience. So one of the sort of ironies of writing this whole book and of this whole project was I got to go to these amazing places and see the most amazing creatures in the process of looking at how humans are affecting the world.

So you'd sort of go to the ends of the Earth, and yet still what you were looking at was how humans are affecting this place.

GROSS: So how are humans affecting the Great Barrier Reef?

KOLBERT: Well, humans are affecting the Great Barrier Reef. In this particular case what they were looking at was how this input of CO2, so how the changes in ocean chemistry, people had measured the rate at which the reef was sort of you could say putting on weight, so growing, a couple decades ago, back in the '70s.

And they were looking at the rate at which the reef was growing now. And one of the impacts that's predicted from ocean acidification, just due to sort of basic, you know, chemistry, you don't even have to do any experiments really, is that it's going to be harder and harder for any organism that makes a shell or an external skeleton like a coral out of the mineral calcium carbonate. It's going to get harder and harder for them to do that.

And that is exactly what people are finding. That's what they found on One Tree Island, that the rate at which the reef was growing has declined substantially. And as this process continues, so as the ocean acidification increases, that rate is going to slow and slow until the point, which is unfortunately not very far in the future, where it seems that corals will no longer be able to keep up with basically the forces that are trying to wear them down, which is everything from organisms that eat away, fish that eat away at the reef and just wave action.

And then you're looking at the basically collapse of the whole ecosystem, the reef ecosystem, which supports an extraordinary array of creatures, as anyone who's ever seen, you know, any underwater footage of a coral reef knows, and as I saw when I was out on the Great Barrier Reef, it's just this extraordinary, you know, Jacques Cousteau-esque picture that unfolds under your eyes when you're looking down into the water.

GROSS: So this is going to sound like a horrible question, but, you know, I don't get to see barrier reefs, I don't get to see coral, I live in the city. What impact does it have on my life if coral reefs can't grow anymore and if they start declining because of the acidification of the oceans?

KOLBERT: Well, I guess I'd give you two answers. The first answer is, you know, we are effectively undoing, you know, the beauty and the variety and the richness of the world, which has taken tens of millions of years to reach this point. We're sort of unraveling that. And if that is something that you just say, well, I don't care about, then I guess I'd say, well, what do you care about.

(LAUGHTER)

KOLBERT: But on another, on a more, you know, personal sort of like I want to know how - you know, what's it mean to me, I guess my answer would be we're not sure. You know, no one's - we haven't done this before. You don't get to sort of see this experiment run over and over again. So we're doing, it's often said, a massive experiment on the planet, and we really don't know what the end point's going to be.

GROSS: OK, that's actually a very impressive answer because who knows, right.

KOLBERT: Right, exactly. And I think one of the lessons and one of the points of writing the book, as well, is one of the lessons of the mass extinctions of the past is that very dominant organisms - I mean, the dinosaurs, as people point out, we like to think of the dinosaurs, you know, we even use the word dinosaur to mean some sort of lumbering idiot, you know, something that's obsolete.

The dinosaurs were not obsolete. The dinosaurs were not doing anything wrong. They were just unfortunate. And the rules of the game changed very suddenly, and they were gone. And we're not sure why dinosaurs are gone, but they seem to have had some vulnerability to what happened in this extinction event. And we don't know what the end result of what we're doing is. But the rules - if the rules of the game change, you know, then you could sort of - I guess you could say all bets are off at that point.

GROSS: If you're just joining us, my guest is Elizabeth Kolbert. She writes about science for the New Yorker. Her new book is called "The Sixth Extinction: An Unnatural History," and it's about how global change and other manmade changes to the environment are, scientists say, creating a new mass extinction. Let's take a short break here; then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR, and if you're just joining us, my guest is Elizabeth Kolbert. She writes about science for the New Yorker. Her previous book was about global warming. Her new book is about how the impact of global warming and other manmade changes to the earth and the atmosphere are creating what scientists are calling a sixth extinction, a mass number of species has become or is on the verge of becoming extinct.

No one knows exactly where this will lead, but her book is about ongoing experiments and studies to try to understand this new extinction. And it's also about the histories of the previous mass extinctions.

You write that amphibians are the world's most endangered class of animals and that some scientists argue that an extinction that is catastrophic for amphibians is already underway. So let's start with a real basic question here. Like explain what comes under the category of amphibian.

KOLBERT: Well, frogs, toads, salamanders and this very, very interesting, weird, limbless class of creatures that we very rarely see in the temperate zone and that in fact you very rarely see even in the tropics called caecilians. Those are the three groups of amphibians.

GROSS: And I've been reading about, like, frog extinctions, mysterious, like, mass frog deaths for years. And were frogs among the first of the amphibians to really be noticed as dying off?

KOLBERT: Yes, absolutely, what's become known as the amphibian crisis first came to the notice of scientists when people went out in places that they had always found, you know, huge numbers of frogs and toads, who are, you know, related, and just couldn't find them, just literally could not find them, places where - you know, in for example in the Sierras, one scientist told me that he went to a place where in his own youth, in his own graduate student days, you could go up there, and you couldn't avoid these frogs. You could practically not avoid stepping on them.

And they went up there, and they just couldn't find any.

GROSS: In fact you went to a place that used to be filled with golden frogs, and they've disappeared.

KOLBERT: Yeah, that's a very fascinating and amazing story, which is this is in Panama, in central Panama. There's a frog known as the Panamanian golden frog. And it's a beautiful, beautiful frog. It's sort of a taxicab yellow frog with very skinny arms and legs. And it's considered a good luck symbol. It was often printed on, like, lottery tickets in Panama. And there were places that you could go where these yellow frogs were just all over.

One herpetologist described it to me as just crazy. You know, they were just everywhere sunning themselves. They like to sun themselves. And then this scourge came through, which turns out to be a fungus, a fungal disease, and just wiped them out. And what happened is that scientists realized because they had seen this pattern before, they actually anticipated this, and they took some of them out of the forest into a conservation center in central Panama, just like basically an ark.

And they are breeding them there. They're still there. There are still Panamanian golden frogs in this center, but they cannot go back out into the world, into their own native habitat, because this fungus is still there, and the fungus survives even though the frogs no longer do.

GROSS: Now is the fungus a result of human activity?

KOLBERT: Well, that is - certainly seems to be the case because this fungal disease appeared, interestingly enough, in many really disparate parts of the world at the same time, and that's what really had people scratching their heads back in the early '90s. You know, how do we get this - these amphibians disappearing in these often very untouched, seemingly untouched places but in very different parts of the world, you know, in South America, in Australia, you know, oceans away.

And so people have surmised that it must have been something that was moved around the world by people because, you know, a fungus really can't cross the oceans. And one theory, it's been very difficult to pin down, but one theory is that this fungus was moved around the world, another really interesting story, on frogs that were used in the '50s and '60s for pregnancy tests, something called the African clawed frog.

If you inject it with the urine of a woman who's pregnant, it will lay eggs very quickly, and obstetricians used to keep whole tanks of these frogs in their offices, and the African clawed frog turns out to be a frog that carries this fungus but doesn't seem to be killed by it.

So one theory is that as these frogs were imported, or exported I guess you'd say, all around the world, they carried this fungus with them. They were subsequently sometimes maybe, you know, let go by an obstetrician who didn't want them anymore. They established themselves in various places, and maybe they brought it around the world. So we brought the frogs, and the frogs brought the fungus.

GROSS: Wow.

(LAUGHTER)

KOLBERT: Yeah, it's...

GROSS: I didn't even know that - I've heard of the rabbit test, where you injected a woman's urine into a rabbit to see if she's pregnant. I've never heard of the frog test.

KOLBERT: The frog test. The frog test was very widely used. And if you talk to people who, you know, were around then, you know, there are people who still remember, you went to your obstetrician, and there was a tank of frogs.

(LAUGHTER)

KOLBERT: Yeah, I know. It sounds - but it's also one of this...

GROSS: Excuse me.

(LAUGHTER)

KOLBERT: Yeah, yeah, it's one of those things - talk about unintended consequences to cause a major die-off of a really, really ancient group. I should say amphibians have been around for a really, really long time.

GROSS: Yeah, you say amphibians, like they survived the other extinctions.

KOLBERT: Yeah, they're tough. They're survivors. They predate, you know, the dinosaurs. And they're still around. And they're being really, really, really hard-hit right now.

GROSS: So there's a lot of different species of frogs that have been hit by this, not just the golden frogs that you went to see.

KOLBERT: Yeah, there are dozens of species that were probably done in by this fungus, many of them these really colorful tropical frogs that are just beautiful.

GROSS: Elizabeth Kolbert will be back in the second half of the show. Her new book is called "The Sixth Extinction." I'm Terry Gross, and this is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. I'm Terry Gross back with Elizabeth Kolbert. We're talking about her new book, "The Sixth Extinction." Scientists believe the fifth extinction - which killed the dinosaurs - was caused by an asteroid. But in the epoch we're living in, the sixth extinction, it's human behavior that's responsible for mass die-offs of plants, animals and insects. As we're about to hear, one of the contributing factors is human travel.

Kolbert is a staff writer for The New Yorker. Her previous book was about climate change.

Because we are so mobile, especially with air travel now, we've been moving around plants and animals, insects and bacteria and viruses, and we've been doing this voluntarily and involuntarily in our travels, totally, you know, reshaping certain aspects of the ecosystems of the world. And you say that scientists are describing the era we're in as the New Pangaea. Tell us what the original Pangaea was.

KOLBERT: The original Pangaea - and this is, you know, taking a very long view here -the original Pangaea was a supercontinent, it's called a supercontinent. And it was this interesting moment in geological history about 300 million years ago to about 200 million years ago - let's say very roughly - where all of the world's land masses sort of due to plate tectonics just, you know, cruised into each other and crashed into each other and you had basically all the land in the world in one clump. And that was Pangaea.

GROSS: And why is this being called the New Pangaea?

KOLBERT: Well, it's, if you take the very long view once again, then you realize that since Pangaea began to break up, all of these evolutionary lineages have been evolving separately on separate continents for, you know, roughly 200 million years, let's say. And so they've taken different paths. This is something that, you know, was very important to Darwin. Darwin looked at that different continents and he saw that they had different fauna, even though, say, their climate and their terrain were very similar, so parts of South America were very similar in that sense to parts of Australia, but they had completely different flora and fauna and that was because they've been evolving separately for such a long time. He didn't understand plate tectonics but he knew that they'd been separated - that they were separated.

And now what we're doing, if you are taking the flora and faunas, so the plants and animals of South America and you're moving them to Australia and vice versa - and we see this all around, you know, lots of Asian species in the U.S. now, lots of North American species in Europe, lots of European species in Australia. You're effectively recombining the continents and bringing these evolutionary lineages back together, and very, very quickly, from one day to the next, OK? So after, you know, tens of millions of years of evolving separately, we say, OK, one day we're going to, you know, see what happens when we throw you together. And when you do do that, it turns out sometimes nothing happens, but sometimes very, very catastrophic things happen.

GROSS: Give us an example of something catastrophic.

KOLBERT: Well, a good example often comes from islands, day. So there's a very famous example on the island of Guam. Something called the brown tree snake was introduced from New Guinea probably during World War I in military cargo. It was brought to Guam and it went absolutely, you know, sort of berserk. It had no enemies. One thing that happens when you move something around is it probably may not have any natural predators, so the brown tree snake had no predators in Guam, and Guam itself lacked any animal and it's only snake was this, you know, tiny little wormlike thing. And so it's birds, there were a lot of bird that fell victim to it. It's a very, very voracious snake, it turns out. It went wild, multiplied until there were something like 40 snakes per acre, killed off a lot of the island's native birds, a lot of reptiles were affected, the bats were affected, so the snake just came in and took what's sometimes called naive populations. This is especially true on islands and basically wiped them out.

GROSS: The idea of humans moving around species and bacteria and viruses and so on to places where they didn't naturally occur, that's nothing new. I mean there's something that's now known as the Columbian Exchange, like when Columbus and other explorers went to different parts of the world - including the Americas - they brought plants and animals with them to the continents that they were exploring and then brought back home plants and animals from the continents they had explored. So this whole idea of moving around the ecosystem is centuries old.

KOLBERT: Absolutely. This is not, I mean a lot of, one of the points I also, you know, sort of hope comes through in the book is this is not a new project that humans are embarked upon. And exactly, we can go back to pre-Colombian times. We can go back to for example, people reached Australia, you know, 50,000 years ago - which is an ,amazing fact that people reached Australia 50,000 years ago. But they found this sort of amazing cast of huge animals - huge birds, huge kangaroos, huge wombat-like creatures that are known as rhinoceros wombat that sort of look like, you know, enormous guinea pigs, let's say, and they are no longer there. You know, so people have been at this world-altering project for quite a while now. And when the first, in terms of bringing species around the world, for example, rats were brought with very early Polynesian settlers in the Pacific, so they were brought to, you know, places like Fiji and they wreaked havoc. They wreaked absolute havoc already, you know, say, 1,500 years ago. So you're absolutely right, this is not a project that we've suddenly embarked upon, but I think what has changed is the scale and the rate. For example, it's been estimated that just in ship ballast now in our enormous supertankers, people are moving 10,000 species a day around the world. So even though our ancestors, you know, were already at this project, and did caused significant effects because of that and significant numbers of extinctions, we have just ramped it up to a whole new level.

GROSS: If you're just joining us, my guest is Elizabeth Kolbert. She writes about science for The New Yorker. She's the author of a new book called "The Sixth Extinction," and it's about what many scientists believe is a mass extinction that is now underway, that is as important as previous extinctions, like the mass extinction of dinosaurs. Kolbert's previous book was about climate change.

Elizabeth, let's take a short break and then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. My guest is Elizabeth Kolbert. She writes about science for "The New Yorker." Her new book is called "The Sixth Extinction," and it's about what many scientists believe is a mass extinction that we are beginning to experience, that's affecting life in the ocean and on land. Her previous book was about climate change. She writes about science for The New Yorker.

You know, when we look at the big picture about how ecosystems are changing and how many, you know, species are becoming extinct, if you look at the big picture, there's the catastrophe, you know, whether it's like, you know, a glacial - glacially caused die-off or an asteroid, you know, caused series of extinctions or, you know, climate change, global warming, but then there's also, you know, evolution. Evolution is a very, very slow process of adaptation and catastrophe is a much kind of quicker and sudden dramatic change. So how did catastrophe and evolution coexist or balance each other or not balance each other?

KOLBERT: Well, that's a really, I mean really interesting question, and how people have come to look at the whole history of the world as this interesting - as I quote one paleontologist describing it, as long periods of boredom interrupted occasionally by panic.

(LAUGHTER)

KOLBERT: And the history of, you know, how we came to see the world this way, that is the history of science in that sense, that's part - that's a story that I sort of open the book with in a way. And that dates all the way back to the beginning of the 19th century when the whole history, the whole concept of extinction was in a way discovered. And what's interesting is before that, people really didn't even think there was such a thing as extinction. They couldn't imagine that the creator would create something that wouldn't be around forever. So when people started to find really interesting things, mastodon bones, mammoth bones, they just assumed that those creatures were out there somewhere. And in fact Thomas Jefferson, when he sent Lewis and Clark out to the American Northwest, he really hoped that they were going to find live mastodons roaming around out there because he really could not accept the fact that they were gone.

And it was a French naturalist who said, you know, that's ridiculous. Obviously they're gone, these creatures are extinct. And he really - his name is Georges Cuvier, he's a very fascinating character. He really invented - as it were - the concept of extinction. And he thought all extinctions were caused in these catastrophic changes. And then Darwin and his - Darwin's mentor, a man named Charles Lyle, came along, and they completely dismissed the idea that you could have catastrophes. They decided that the Earth changed very, very slowly and only very slowly. They made a slight sort of logical leap there. It changed slowly and it could only change slowly. So the idea of catastrophe really was discredited. We just didn't get events like that. And it was only pretty recently, in the '80s, when it was discovered by a scientist named Walter Alvarez, who is out of Berkeley, that an asteroid impact was what had done in the dinosaurs, and that was extremely controversial at the time and now very well accepted, that this concept that we could get catastrophic change on the Earth was accepted, and now I think the general sort of sense, which, you know, is sort of a hybrid of the two, is that, yes, the world changes very, very slowly, except at certain moments when it doesn't. And it seems that we are in one of those moments when it is in fact changing very, very fast and, of course, we are the agent for that.

GROSS: So when you look at the mass extinctions, what do scientists say about the odds that humans are going to survive?

KOLBERT: Well, I...

(LAUGHTER)

KOLBERT: I sort of pointedly try to avoid that question. I mean I think if you were, you know, saying, well, what's going to survive an event that humans have set in motion, I guess you would bet on humans. We're very, very clever. We're very technologically able. There are a lot of us. There's, you know, right now about 7.2 billion of us and if we continue on the trajectory we're on, you know, there are one of the nine or 10 billion soon. So if you were sort of just betting what creature, you know, would survive this, I think you would bet on humans. But as I said before, one of the very, very sobering things about looking back at the mass extinctions of the past is that when the rules change you don't really know what's going to happen. So I guess you'd say if you were a species, you know, causing a mass extinction, you wouldn't necessarily feel entirely secure.

GROSS: One scientist is predicting that it's rats who will one day take over the Earth.

KOLBERT: Yeah. That's a wonderful, I mean I should say that's part...

GROSS: Did you say wonderful?

(LAUGHTER)

KOLBERT: Well, it's a wonderful, it's a wonderful idea. It's a very graphic, vivid idea, I think, that captures people's imagination. And I should say it's partly tongue-in-cheek, but partly not. And that idea comes from a geologist that I went out with named Jan Zalasiewicz. And he is in fact leading this whole effort to assess whether we should formally name a new geological epoch, whether we live now in the Anthropocene, and he has written about looking into the future. Because part of what we're doing, which is, you know, interesting and also very sobering, is we're closing off certain evolutionary paths, but we're also opening up certain evolutionary paths, right? So what is going to populate the planet millions and millions of years from now is going to be what could co-exist with humans, and then what produced descendents. And his point is, look, if you look at rats, which followed people around from very early on, you know, first the Polynesians, then the Europeans brought a different species of rat to a lot of these places. So there are now rats on islands that never ever had rats before. And they've done incredibly well, you know, they've often eaten through a lot of other species. And his point is they are obviously survivors. They do very well in areas of human disturbance. And maybe their descendents, they could diversify. They could become big. They could become tiny. They could lead to all sorts of new species in the future. And he points out in one of his books let's imagine just for the sake of argument a rat that lives in a cave, clothes itself in the skin of other mammals, and that reminds us of a species that we all know.

GROSS: Your previous book was about climate change. That came out in 2006 and that book is very related to your book about the sixth extinction because climate change is such a factor in the extinctions that we're already starting to see. But when you look back eight years ago to when the book was published, what changes have you seen in that interim connected to climate change?

KOLBERT: Changes you mean to the physical planet?

GROSS: To the physical planet. It's a very short amount of time.

KOLBERT: Yeah.

GROSS: But, you know, a lot has happened. A lot of measurable things have happened in that eight years.

KOLBERT: Absolutely. Yeah. Well, one thing that's happened, I think maybe the most vivid thing that's happened was when I was writing that book people were looking at the Arctic sea ice and I went to talk to people who studied the Arctic sea ice. And I went to see the Arctic sea ice. And when I was writing the book - so that's, you know, roughly a decade ago - there were climate model predictions saying based on, you know, what we see in the models we think that the Arctic Ocean could be ice free in summertime. So there'd be no ice cap in the Arctic Ocean in summer by towards the end of this century.

And since then the Arctic ice cap has melted at such an astonishing rate that you now have people saying we could have an ice-free Arctic Ocean, so the North Pole would just, you know, be open water in the summertime, within the next few decades. So that's one change that has happened, you know, way faster even than people anticipated and a very, very dramatic one.

GROSS: What's your reaction when you see people debating whether climate change actually exists and whether it's man - whether it has been created by, you know, human activity?

KOLBERT: Well, I guess my reaction is, you know, frustration on the one hand and sadness on the other because it's just evidently clear what's going on. The physics of it - the geophysics of it are abundantly clear. They were already understood, you know, back in the 1850s and in the 1890s you already had the scientist - he was in fact a Nobel Prize winning scientist named Svante Arrhenius made predictions about what was going to happen if we kept, you know, adding CO2 to the atmosphere.

Now, he was off on his predictions in a lot of ways, in fact, because we've added CO2 - in part because we've added CO2 to the atmosphere so much faster than he could even have imagined. So the fact that we're sort of debating what the effects of adding CO2 to the atmosphere are, you know, 150 years after people understood the effects of adding CO2 to the atmosphere is sort of sad.

And what we should be debating, to the extent that we should be debating anything, is what we should do about it. We shouldn't be debating whether this is having an impact because as you see the Arctic ice cap melt away which is, you know, exactly what was predicted by climate models, even if they were somewhat too conservative, you shouldn't really be debating anymore whether climate change is having an effect.

GROSS: So I think a lot of people will say, well, look. This is the sixth extinction. So there were five others. They killed off the dinosaurs and, you know, a lot of other, you know, animals that we would call prehistoric. So another extinction? That's probably part of nature's course. There's already been five extinctions so why should we be panicking about a sixth? You know, this is obviously the way the cycle works.

(LAUGHTER)

KOLBERT: Well, if you want to take that sort of philosophical view, the grand, grand philosophical view, I guess on some level I say, OK, go for it. But I want to point out that in a mass extinction, one of the things that seems to happen is that the whole rules, you know, of life change and so you don't know what's going to come out at the end of that.

And another point that I would make to people who want to take, you know, just the sort of 500 million yearlong view is after a mass extinction it often takes about - or it has generally tended to take - many millions of years for life to recover. It's not something that, you know, you bounce back from just from one day to the next.

So I don't really think, you know, given a choice people would say well, that's the period I want to live in, a period of mass extinction.

GROSS: Thank you so much for talking with us.

KOLBERT: Oh, thanks for having me.

GROSS: Elizabeth Kolbert is a staff writer for The New Yorker. Her new book is called "The Sixth Extinction." You can read the prologue on our website freshair.npr.org. This is FRESH AIR. Transcript provided by NPR, Copyright NPR.